Pcm modem over all-digital connection

ABSTRACT

A method of transmitting data. The method includes establishing a modem connection ( 72 ) between two digitally connected modems ( 24, 28 ) and transmitting data on the modem connection ( 82 ) in accordance with a transmission scheme defined only for transmission from analog to digital modems.

RELATED APPLICATIONS

[0001] The present application is a continuation-in-part (CIP) of PCT application PCT/IL00/00198, filed Mar. 29, 2000, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to communication systems and in particular to voice band modems.

BACKGROUND OF THE INVENTION

[0003] Voice band modems (VBMs) are used for transmitting data over telephone communication links. Generally, two modems on opposite ends of a communication link form a modem connection and send each other data on the connection by converting (i.e., modulating) the data into electrical signals suitable for transmission on the link. Generally, the modems of a link are referred to as a client modem (e.g., of a home user) and a server modem (e.g., of an Internet service provider). One of the modems, referred to as the call modem, generally dials the telephone number of the other modem in order to form the connection. In most cases, the call modem is the client modem, however in some cases the server modem acts as the call modem. Existing telephone links include analog lines, such as twisted copper pairs through which most residential homes are connected to the telephone network, and/or digital trunks, such as E1 and T1 links.

[0004] In order to allow modems of different vendors to transmit data to each other, standards (also known as protocols) have been defined stating exactly how the signals should be modulated by the modems. Usually modems implement a plurality of different standards. Generally, when two modems form a connection, they search, in a protocol negotiation phase (e.g., a V.8 stage), for a highest transmission rate protocol which they both implement, and use this protocol for transmission over the connection.

[0005] The V.34 standard describes a commonly used transmission protocol, which allows transmission of signals at rates of up to 33,600 bps. The V.34 standard is described in ITU-T Recommendation V.34, 2/98, the disclosure of which is incorporated herein by reference.

[0006] The end modems of the V.34 standard may be connected to the telephone link through a digital trunk, in which case the modem is referred to as a digital modem, or may be connected through an analog loop, in which case the modem is referred to as an analog modem. A digital modem provides samples at a rate of the digital trunk, generally 8000 samples per second. The analog and digital V.34 modems are substantially the same except that the linear data representation used in the modem is converted to PCM samples e.g., G.711 samples, for the digital modem and into analog symbols for the analog modem. The provisions of the V.34 do not differentiate between the direction of transmission and the same provisions are used for the upstream (from the client to the server) as for the downstream (from the server to the client).

[0007] In order to achieve higher transmission rates, the V.90 standard, described in ITU-T Recommendation V.90, 9/98, the disclosure of which is incorporated herein by reference, was defined. The V.90 standard imposes the limitation that the server modem is connected to the communication link through a digital trunk. Using this limitation, the downstream transmission rate is increased to about 56 Kbps. The V.90 standard allows for the transmitter of the digital modem to perform acts that compensate for analog line conditions, for example, spectral shaping which helps the receiver of the client modem to combat the effects of an intermediate digital to analog conversion. During a training phase, the client modem instructs the digital modem, as to which spectral shaping parameters are to be used. Generally, in the V.90 standard, the client modem is referred to as an analog modem, which is connected to the telephone link through an analog loop. In defining the first phase of the negotiation stage (the V.8 protocol negotiation), the V.90 standard states that when both modems identify themselves as being digitally connected to the telephone link, the call modem shall operate as the analog (client) modem and the answer modem shall operate as the digital (server) modem. Similar statements appear in the V.8 ITU recommendation (2/98), the disclosure of which is incorporated herein by reference. It is noted, though, that the V.90 and V.8 standards do not state any provisions for the operation of a client modem, digitally connected to the link, as an analog modem, thus possibly leaving the definition from the V.8 negotiation stage as a theoretical provision for farther development.

[0008] While the V.90 standard improves the downstream transmission rate, it leaves the upstream transmission rate as in the V.34 standard. In order to improve the upstream transmission rate of the V.90 standard, the V.92 standard was introduced. The V.92 standard is described in V.92 ITU draft PCM-00-062R1, the disclosure of which is incorporated herein by reference. The V.92 standard uses the downstream provisions of the V.90 standard but introduces a new method for upstream transmission so as to achieve upstream transmission rates of up to 48 Kbps. In order to achieve these upstream transmission rates, a third phase of the connection training in the V.92 standard determines the effect of the analog transmission link, on the uplink transmission, to allow compensation for the determined effect.

[0009]FIG. 1 is a schematic time chart of training signals transmitted according to a portion of the third phase of the V.92 standard, as is known in the art. Generally, only information required for the explanation of the present application are described herein, while the details of the signals shown in FIG. 1, appear in the V.92 standard. Responsive to receiving a TRN_(1d) signal 500, the analog modem transmits an S_(u) signal 502, used for evaluation of the analog line, for a period of 144T (T being the sample transmission interval). That is, the analog modem transmits 144 samples of S_(u) signal 502 at a sample rate of 8000 samples per second. Thereafter, the analog modem transmits an S_(u)-not signal 504 for a period of 24.5T, followed by transmission of S_(u) signal 506 until a J_(p) signal 508, which includes a phase correction factor (ε), is received from the digital modem. When J_(p) signal 508 is received, the analog modem transmits a second instance of S_(u)-not signal 510 for a period of (24+ε)T. The transmission of signals with fractional intervals (i.e., intervals which include a non-integer number of sample periods), for example, S_(u)-not signal 504 and S_(u)-not signal 508, and the consequent transmission of out of phase signals, e.g., S_(u) signal 506, generally requires the use of an analog modem, for example using a resampler and/or a digital to analog converter of the analog modem.

[0010] In recent years, increasing numbers of computers connect to ISPs through all-digital connections, for example, using cellular phone connections. The V.91 standard, described in ITU-T Recommendation V.91, 5/99, the disclosure of which is incorporated herein by reference, was defined for such connections, allowing rates of 64 Kbps in both directions. The V.91 standard, however, is not widely implemented. If one of the modems of an all-digital connection does not support the V.91 standard, contemporary modems will generally fall back to using the V.34 standard, which provides a maximal transmission rate of 33.6 Kbps in both the upstream and downstream directions, as the V.90 and V.92 standards are not implemented for all-digital connections.

[0011]FIG. 2 is a schematic illustration of a modem connection as is known in the art. A client modem 12, connected to a public switching telephone network (PSTN) 19 through an analog loop 14, forms a connection with a server modem 24 which is connected to PSTN 19 through a digital trunk 32. Usually, modem 24 belongs to a modem pool, for example of an Internet service provider (ISP) 29. Generally, a line card 16 translates the signals from analog lines 14 to digital link 36 of PSTN 19, and vice versa. It is noted, that in most cases, except for analog loop 14 which connects line card 16 and client modem 12, PSTN 19 is formed of substantially only digital links, represented in FIG. 2 by a digital network 23.

[0012] Client modem 12 comprises a signal processing unit 13 and a sampler and reconstructer 15 which turns digital signals into analog signals for transmission and analog signals from lines 14 into digital signals. When a computer 10 requests to connect to ISP 29, client modem 12 forms a negotiation V.8 connection with server modem 24. During the negotiation connection, client modem 12 identifies as an analog modem and server modem 24 identifies as a digital modem such that the modems agree to use a V.90 connection for transmission of data. Thereafter, modems 12 and 24, sequentially as defined by the V.90 protocol, transmit test signals used to check the characteristics of loop 14 (and trunk 32).

[0013] After the tests are concluded, modems 12 and 24 move into transceiving states according to the V.90 standard. Signals transmitted from computer 10, are prepared for transmission by processing unit 13 of modem 12 at a rate of up to 33.6 Kbps and are then converted to analog signals by a D/A of sampler and reconstructer 15. Signals transmitted by server modem 24 are transmitted from the modem at a rate of up to 56 Kbps as described above. The transmitted signals from server modem 24 are converted to analog signals by line card 16 and are passed on link 14 to modem 12. An A/D of sampler and reconstructer 15 of modem 12 samples the analog signals at a high enough rate which allows proper operation of the modem, i.e., proper synchronization of a clock of sampler and reconstructer 15 in receiving modem 12 to the timing of transmitting modem 24. Generally, to allow for rate correction, the A/D of sampler and reconstructer 15 samples the signals at a rate higher than 8000 samples per second. The sampled digital signals are then passed to processing unit 13 for processing.

[0014]FIG. 3A is a schematic block diagram of an exemplary data receiving path 40 of processing unit 13 of modem 12, as is known in the art. Path 40 may be used, for example, for the V.90 and V.92 protocols. Path 40 receives samples from sampler and reconstructer 15 on a line 42. The samples are added to correction values provided by an echo canceller 44 and are then filtered by a channel filter 46. The filtered samples are passed to a timing recovery unit 48 and a rate converter 50, which correct for timing drifts of the received samples. The samples are passed through an automatic gain control (AGC) unit 52 and are then provided to an equalizer 54 which corrects phase and amplitude distortions of the received samples. The samples from equalizer 54 are passed to a symbol decision module 56 which determines for each sample which symbol it represents. Symbol decision module 56 also detects attenuation pad impairments and performs robbed bit signaling (RBS) in order to better perform the determination of the samples which represent the symbols. The symbols are then passed through a symbol to bit converter 58 which translates the symbols into bits, and through a descrambler 60 which descrambles the bits, which were scrambled by server modem 24 before their transmission.

[0015]FIG. 3B is a schematic block diagram of an exemplary data transmission path 61 of processing unit 13 of modem 12, as is known in the art. Path 61 may be used, for example, for the V.92 protocol. Transmission path 61 includes a modulus encoder 63 and a precoder 64, which convert data bits to be transmitted into transmitted symbols. As is known in the art, an inverse mapper 65 and a convolution encoder 66 participate in generating the symbols with precoder 64. The generated symbols are passed to a prefilter 67, which filters the signals before they are transmitted onto a line 62, to sampler and reconstructer 15 (FIG. 2).

SUMMARY OF THE INVENTION

[0016] An aspect of some embodiments of the present invention relates to a digitally connected modem which is adapted to operate as a V.92 analog client modem.

[0017] An aspect of some embodiments of the present invention relates to a digitally connected modem which is adapted to transmit signals in accordance with a transmission scheme defined only for transmission from analog to digital modems. Optionally, the transmission scheme comprises the upstream transmission method of the V.92 protocol.

[0018] An aspect of some embodiments of the present invention relates to a digitally connected client modem adapted to transmit one or more training signals for an interval having a length of a non-integer number of samples, e.g., the S_(u)-not signal of the V.92 protocol.

[0019] In some embodiments of the invention, the digital client modem comprises a memory which stores a sample value which represents both a fractional portion of the signal transmitted for the non-integer interval and a complementary portion of a signal transmitted immediately thereafter. Optionally, the fractional portion comprises half of a sample. Alternatively or additionally, the modem resamples the training signal at a higher rate, for example twice the original rate, and immediately down samples the signal. At the interface between the transmission of the signal transmitted for the non-integer interval and the signal transmitted immediately thereafter, the down sampling skips one or more of the higher rate resampled samples, so as to generate a sample which represents fractional portions of both the signals.

[0020] An aspect of some embodiments of the present invention relates to a digitally connected client modem adapted to transmit two instances of a training signal, separated by an interval corresponding to a non-integer number of samples. Thus, the modem is adapted to transmit the training signal in two different instances shifted relative to each other by a plurality of symbols plus a fraction of a symbol, for example half a symbol. In an exemplary embodiment of the present invention, the training signal comprises the S_(u) signal of the V.92 protocol and the interval between the instances is 24.5 samples.

[0021] In some embodiments of the invention, the digital client modem comprises a memory which stores the two instances of the training signal, optionally in the form of two series of samples. Alternatively or additionally, the training signal is cyclic and the memory stores for each of the shifted versions of the training signal, a series of samples representing a single cycle of the signal. In transmitting the training signal, the modem uses the series corresponding to the required shift amount. Further alternatively or additionally, the memory stores a table which includes the samples from which the signal is formed, for each of the required shifts of the signal. In generating the training signal, the modem optionally accesses the table based on the required shift and the current position within the training signal. Alternatively or additionally, the modem resamples the training signal at a higher rate, for example twice the original rate, and immediately down samples the signal with the required shift.

[0022] An aspect of some embodiments of the present invention relates to a modem adapted to receive, during a modem training phase, a message which includes a value to be used in determining one or more parameters of a transmitted training signal. The receiving modem ignores the value in the message and uses predetermined values for the one or more parameters.

[0023] In some embodiments of the invention, the one or more parameters include a fractional transmission time of the training signal the modem is to transmit, e.g., as in the J_(p) message of the V.92 protocol. The training signal is transmitted with a predetermined fractional transmission time, optionally with a fractional transmission time of half a symbol. In some embodiments of the invention, the modem comprises a digital modem, which due to its digital connection does not cause phase shifts and therefore the fractional transmission time in the directive is generally half a symbol.

[0024] An aspect of some embodiments of the present invention relates to a digitally connected modem which performs, for one or more protocols, fewer tasks than required by an analog connected modem, for the protocol. Optionally, the one or more protocols differentiate between analog and digital modems. Alternatively or additionally, the one or more protocols state different provisions for the upstream and downstream transmissions.

[0025] Optionally, the fewer tasks are performed in the transmission path. For example, instead of performing all the tasks performed by an analog connected modem and in addition performing an analog to digital conversion or replacing the last conversion to analog signals by a conversion to PCM signals, the digital modem directly converts 16 bit linear level samples, used in the transmission path, into PCM signals. In some embodiments of the invention, the transmission path does not include one or more filters, for example, a precoder filter and/or a prefilter, which are used in transmission paths of analog modems.

[0026] Alternatively or additionally, the fewer tasks are performed in the reception path. For example, instead of performing all the tasks performed by an analog connected modem on the received samples, the digital modem ignores tasks not required for digitally connected modems. Optionally, the ignored tasks are tasks required for correction of analog line impairments, such as, echo cancellation, rate conversion, channel filtering, automatic gain control, time recovery and/or equalization.

[0027] An aspect of some embodiments of the present invention relates to a digital modem which purposely incorrectly identifies itself as an analog modem during a protocol negotiation procedure, e.g., a V.8 or V.8bis procedure. In some embodiments of the invention, the digital modem selectively determines whether to identify as a digital or analog modem according to a guess of the identity of the remote modem. Optionally, the digital modem identifies as an analog modem, for analog-to-digital modem protocols, e.g., V.90, V.92. Alternatively, the digital modem determines whether to identify as analog or digital according to a user instruction. Alternatively or additionally, the digital modem determines whether to identify as analog or digital according to the telephone number of the remote party of the connection.

[0028] In some embodiments of the invention, the digital modem identifies itself as an analog modem for one or more protocols and as supporting one or more all digital protocols (e.g., V.91) in a single connection. Alternatively, the digital modem consistently identifies itself as an analog modem for all protocols, i.e., it does not state support of all digital protocols.

[0029] By having the digital modem identifying as an analog modem, the digital modem is able to have a remote digital modem agree to form an analog-to-digital connection with the digital modem, for a protocol that is defined only between an analog and a digital modem.

[0030] There is therefore provided in accordance with some embodiments of the invention, a method of transmitting data, comprising establishing a modem connection between two digitally connected modems; and transmitting data on the modem connection in accordance with a transmission scheme defined only for transmission from analog to digital modems. Optionally, transmitting the data comprises transmitting in accordance with the upstream transmission scheme of the V.92 protocol.

[0031] Optionally, the transmission scheme defined only for transmission from analog to digital modems is defined for use concurrently on a same line with an opposite direction transmission scheme which has a higher maximal transmission rate.

[0032] There is further provided in accordance with some embodiments of the invention, a digitally connected modem, comprising a digital line interface adapted to transmit symbols at a predetermined rate defining respective symbol periods; and a signal generation unit adapted to generate, during a training phase of a modem connection, symbols of a pair of instances of a training signal, shifted relative to each other by a phase shift of a partial symbol period, for transmission by the line interface.

[0033] Optionally, the training signal comprises a signal defined by the V.92 protocol.

[0034] Optionally, the phase shift comprises a shift of half the symbol period. Optionally, the signal generation unit comprises an interpolator adapted to convert a generated training signal into a series of samples with a sampling rate higher than that of the signals transmitted by the digital line interface and a decimator adapted to down-sample the training signal to the sampling rate of the signals transmitted by the digital line interface, with a phase shift, when a phase shift is required.

[0035] Optionally, the signal generation unit comprises a memory which stores segments of the training signal with and without the phase shift and the generation of the signals is performed with reference to the segments stored in the memory. Alternatively or additionally, the signal generation unit comprises a memory which stores a sample series representing both the instances of the training signal together with at least one signal transmitted between the instances. Further alternatively or additionally, the signal generation unit comprises a memory which stores a table which includes all the sample values of the training signal with different phase shift values.

[0036] Optionally, the different instances of the training signal have different lengths. Optionally, the different instances of the training signal are shifted relative to each other by a shift period including a non-integer number of symbol periods, the phase shift being equal to a fractional portion of the shift period.

[0037] There is further provided in accordance with some embodiments of the invention, a method of transmitting modem training signals, comprising transmitting, from a first modem to a second modem, a message including a value of at least one parameter of a training signal expected to be received by the first modem, and transmitting, by the second modem, the training signal expected to be received by the first modem, with a predetermined value of the at least one parameter, which is independent of the value of the at least one parameter in the message. In an exemplary embodiment of the invention, transmitting the message comprises transmitting a J_(p) message of the V.92 protocol. Optionally, the at least one parameter is related to a length of the training signal. Optionally, the second modem comprises a digital modem.

[0038] There is further provided in accordance with some embodiments of the invention, a method of establishing a modem connection, comprising establishing a physical connection between a pair of digital modems, selecting a protocol which requires that at least one of the modems of the connection is a digital modem, to govern the transmission on the physical connection, and transmitting, on the established connection, at least one signal defined for evaluating analog effects of the physical connection.

[0039] Optionally, selecting a protocol comprises selecting a protocol which uses different provisions for upstream and downstream transmission. Optionally, transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for evaluating a phase shift caused by the physical connection. Alternatively or additionally, transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for determining a required echo cancellation and/or for channel equalizer training.

[0040] There is further provided in accordance with some embodiments of the invention, a modem, comprising a line interface adapted to receive signals transmitted in a downstream of a modem connection in accordance with a modem protocol which has different provisions for the upstream and the downstream, and a digital interpretation unit adapted to receive samples from the line interface, after passing through fewer than all of an echo canceller, an automatic gain controller, a rate converter, a time recovery unit, an equalization unit and a channel filter.

[0041] Optionally, the line interface is adapted to receive signals transmitted in a downstream of a modem connection in accordance with the ITU V.90 or V.92 protocol. Optionally, the digital interpretation unit is connected directly to the line interface. Optionally, the digital interpretation unit comprises a symbol decision module adapted to receive samples directly from the line interface.

[0042] Possibly, the line interface is adapted to receive symbols at a rate of 8000 symbols per second. Optionally, the line interface is connected to a digital trunk line. Optionally, the digital interpretation unit is adapted to receive samples from the line interface, without the samples passing through any unit for overcoming analog signal effects.

[0043] There is further provided in accordance with some embodiments of the invention, a modem, comprising a line interface adapted to transmit signals in accordance with an upstream of a modem protocol which has different provisions for the upstream and the downstream, on a communication line, and a digital transmission unit adapted to transmit samples through the line interface, without passing the samples through both a precoder filter and a prefilter.

[0044] Optionally, the digital transmission unit is adapted to transmit samples without passing through a precoder filter or a prefilter. In some embodiments of the invention, the line interface is adapted to transmit signals in accordance with the upstream of the V.92 modem protocol. Optionally, the digital transmission unit comprises a constellation point selector which is connected to the line interface without intervening filters.

[0045] Optionally, the digital transmission unit performs calculations using 16 bit linear level samples and the modem comprises a converter, which converts 16 bit linear level samples into PCM samples. Optionally, the line interface is adapted to transmit signals onto a digital trunk.

[0046] There is further provided in accordance with some embodiments of the invention, a method of forming a modem connection, comprising establishing a physical connection between a first and a second modem, and transmitting a message including data on the capabilities of the first modem, from the first modem to the second modem, the message purposely identifying the first modem as being of a different type, having a different capability or being connected differently than the first modem is actually connected.

[0047] Optionally, the first modem is digitally connected and the message identifies the first modem as being analog connected. Optionally, the message identifies the first modem as supporting an all digital protocol. Alternatively, the message does not identify the first modem as supporting an all digital protocol. Optionally, the second modem is digitally connected. Optionally, the message identifies the first modem as supporting at least one protocol which differentiates between analog connected and digital connected modems.

[0048] Optionally, the message identifies the first modem as supporting at least one of the V.90 and V.92 protocols. Optionally, the method includes receiving a user instruction on whether to identify as an analog or digital connected modem and wherein the message identifies the connection of the modem responsive to the user indication. Optionally, the method includes determining whether to identify as analog or digital connected responsive to a telephone number of the second modem and wherein the message identifies the connection of the modem responsive to the determination. Optionally, the message is transmitted during a protocol negotiation procedure. Optionally, the first modem does not perform, during the protocol negotiation procedure, at least one test required to determine analog connection parameters. Alternatively or additionally, the first modem disregards results of at least one test required to determine analog connection parameters.

BRIEF DESCRIPTION OF FIGURES

[0049] Particular non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, in which:

[0050]FIG. 1 is a schematic time chart of training signals transmitted according to a portion of the third phase of the V.92 standard, as is known in the art;

[0051]FIG. 2 is a schematic illustration of a V.90 connection as is known in the art;

[0052]FIG. 3A is a schematic block diagram of an exemplary receiving path of a V.90 or V.92 modem, as is known in the art;

[0053]FIG. 3B is a schematic block diagram of an exemplary transmission path of a V.92 modem, as is known in the art;

[0054]FIG. 4 is a schematic illustration of a modem connection, in accordance with an embodiment of the present invention;

[0055]FIG. 5 is a flowchart of the actions performed by a digital modem in forming a modem connection, in accordance with an embodiment of the present invention;

[0056]FIG. 6 is a schematic illustration of a transmission path for transmission of training signals of the V.92 protocol, in accordance with an embodiment of the present invention;

[0057]FIG. 7 is a schematic illustration of a transmission path for transmission of training signals of the V.92 protocol, in accordance with an embodiment of the present invention;

[0058]FIG. 8 is a schematic block diagram of a receiving path of a modem, in accordance with an embodiment of the present invention; and

[0059]FIG. 9 is a schematic block diagram of a transmission path of a modem, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0060]FIG. 4 is a schematic illustration of a digital modem connection 20, in accordance with an embodiment of the invention. A mobile unit 22, such as a wireless access protocol (WAP) unit, forms a communication connection With a server modem 24 (usually belonging to a modem pool) of an Internet service provider (ISP) 29. The signals transmitted and received by mobile unit 22 are optionally passed over a cellular link 30 to and from a base station 26 of a cellular company. Optionally, the signals to and from mobile unit 22 are passed through a client modem 28 (also generally belonging to a modem pool) associated with base station 26 or with another base station of the cellular company. Modem 28 is connected to modem 24 through a digital network 23, which is optionally formed only of digital links, such as E1 or T1 links. Digital network 23 may belong to a private or public communication network, such as PSTN 19 (FIG. 2), and/or may comprise a plurality of concatenated data links or may be formed of a single link. Modems 28 and 24 optionally connect to digital network 23 through digital lines 36 and 32, respectively.

[0061] It is noted that FIG. 4 is brought by way of example, and the apparatus and/or methods of the present invention may be used advantageously for substantially any other digital modem connections. For example, a computer may connect to client modem 28 through an ISDN connection instead of through cellular link 30.

[0062] In some embodiments of the invention, server modem 24 comprises a standard server modem, which is configured to connect to both digital and analog client modems. Alternatively, server modem 24 is configured to connect exclusively to analog client modems. In some embodiments of the invention, server modem 24 does not differentiate between analog connected client modems and digitally connected client modems, which operate according to the present invention.

[0063]FIG. 5 is a flowchart of the actions performed by modem 28 in forming a modem connection between mobile unit 22 and ISP 29, in accordance with an embodiment of the invention. Upon receiving (70) a request to form a connection with ISP 29 from mobile unit 22, modem 28 optionally establishes (72) a V.8 negotiation connection with server modem 24 of ISP 29. During the negotiations of the V.8 connection, client modem 28 and server modem 24 exchange (74) ability messages (e.g., CM, JM) which state which protocols are supported by the modems. In some embodiments of the invention, client modem 28 identifies itself as supporting one or more analog-to-digital protocols, which optionally use different transmission schemes for the upstream and downstream, e.g., V.90 or V.92. It is assumed herein that server modem 24 supports at least one of the analog-to-digital protocols supported by client modem 28.

[0064] Optionally, client modem 28 identifies itself as an analog modem, e.g., in the access category of the V.8, such that server modem 24, which is a digital modem, will relate to the modem connection as an analog-to-digital modem connection and agree to use an analog-to-digital protocol for the connection. Alternatively, client modem 28 identifies itself as a digital modem, for example, when one or more analog-to-digital protocols state that in case both end modems of the connection are digital modems, the call modem (i.e., the client modem) acts as the analog modem.

[0065] Further alternatively or additionally, client modem 28 determines, before transmitting the ability message, whether the remote modem, e.g., server modem 24, is an analog or digital modem. In some embodiments of the invention, the determination of whether the remote modem is an analog or digital modem is performed by receiving a user indication, from a user knowing or guessing whether the remote modem is an analog or digital modem. Alternatively or additionally, the determination is performed automatically based on a pre-configured table and/or function and one or more parameters of the connection, such as the telephone number of the remote modem. It is noted that the determination is not necessarily always correct. When the determination is incorrect, the modem connection will fall back to using a slower protocol, e.g., the V.34 protocol.

[0066] In some embodiments of the invention, the ability message of client modem 28 includes an indication of support of one or more all-digital protocols, e.g., the V.91 protocol. In these embodiments, the ability message includes a contradiction as it states both digital and analog connection for the same modem. Alternatively, the ability message of client modem 28 does not indicate support of all-digital protocols.

[0067] As is known in the art, the protocol selected for use on the connection is the fastest protocol supported by both server modem 24 and client modem 28. After the V.8 negotiation stage, tests are performed (76) in order to evaluate the quality of link 32 and/or to determine parameters of the connection, in accordance with the selected protocol, as is known in the art. In some embodiments of the present invention, client modem 28 skips (78) tests required to determine analog connection parameters and/or to evaluate analog lines. For example, client modem 28 optionally does not transmit test signals used to estimate echoes which are to be canceled, e.g., the MD signal of the V.90 protocol, and/or signals used to perform channel equalizer training, e.g., the PP signal of the V.90 protocol. Alternatively or additionally, client modem 28 transmits signals related to non-required tests although the results of the tests are ignored, so that server modem 24 is not confused and/or does not realize that client modem 28 is not an analog connected modem.

[0068] In some embodiments of the invention, client modem 28 ignores results of tests, and/or test signals, conveyed from server modem 24, when the tests relate to analog connections and therefore the results of the test are known or irrelevant. For example, the test results may include a parameter to be used in determining a training signal to be transmitted by client modem 28. In an exemplary embodiment of the invention, when during implementing the V.92 protocol client modem 28 receives the J_(p) signal transmitted by server modem 24, client modem 28 ignores the transmitted value for ε and uses a value of ½, which is the expected value for a digital connected modem.

[0069]FIG. 6 is a schematic illustration of a transmission path 100, of a digital client modem, for transmission of training signals of the V.92 protocol, in accordance with an embodiment of the present invention. Transmission path 100 comprises a data generator 102 and a symbol generator 104, which generate training signals as required by the V.92 protocol, e.g., the S_(u) and S_(u)-not signals. A resampler 106 and a D/A converter 108 convert, under the control of a clock 110, the generated symbols into analog signals, which are transmitted on an analog line 114. An analog to digital converter (A/D) 112 reconverts the signals into a digital form, e.g., PCM samples, in which the signals are transmitted on digital network 23.

[0070] As described above with reference to FIG. 1, during phase 3 of the V.92 protocol, transmission path 100 is required to transmit a first instance of the S_(u) signal for 144 samples, the S_(u)-not signal for 24.5 samples and then transmit a second instance of the S_(u) signal which has a half-sample phase shift relative to the first instance of the S_(u) signal. Optionally, the half-sample shift is performed by resampler 106 and/or D/A converter 108 using methods known in the art.

[0071]FIG. 7 is a schematic illustration of a transmission path 150, of a digital client modem, for transmission of training signals of the V.92 protocol, in accordance with another embodiment of the present invention. Transmission path 150 comprises data generator 102 and symbol generator 104, which generate training signals, substantially as in transmission path 100. Unlike path 100, however, path 150 does not include D/A converter 108 and A/D converter 112, which add to the cost of the modem 28 and are not really needed. Instead, transmission path 150 optionally comprises an interpolator 152 and a decimator 154, optionally with 1:2 and 2:1 rates, respectively. When a phase shift is required, e.g., in the interface between S_(u)-not signal 504 and S_(u) signal 506 (FIG. 1), decimator 154 will skip one of the output samples of interpolator 152 or will use one of the output samples twice. Alternatively, decimator 154 operates at the interface between signals 504 and 506 on one interpolated value from signal 504 and a second interpolated value from signal 506, thus forming a decimated value formed from fractions of both S_(u)-not signal 504 and S_(u) signal 506.

[0072] Referring also to FIG. 1, in some embodiments of the invention, interpolator 152 and decimator 154 are used only during phase 3 or only during the transmission of the S_(u) signal and the S_(u)-not signal. Optionally, the use of interpolator 152 and decimator 154 is terminated after the completion of the transmission of the second instance of S_(u)-not signal 510 (FIG. 1) or responsive to receiving the J_(p) signal transmitted by server modem 24. Alternatively, interpolator 152 and decimator 154 are brought into use immediately before the phase shift is required.

[0073] Alternatively or additionally to using interpolator 152 and decimator 154, data generator 102 and/or symbol generator 104 generate signals S_(u) and S_(u)-not using pre-stored samples of the signals. Optionally, modem 28 stores a series of samples of the entire signal to be transmitted including the phase shifted portions, which are prepared at the time of manufacture of the modem and/or at a later initialization process. In an exemplary embodiment of the present invention, data generator 102 stores a first sample series covering S_(u) signal 502, S_(u)-not signal 504 and a long interval of S_(u) signal 506 and a second sample series covering S_(u)-not signal 510. The stored length of S_(u) signal 506, is optionally longer than ever expected to be transmitted. When S_(u) signal 502 is to be transmitted, data generator 102 transmits the stored signal until J_(p) signal 508 is received, and then transmits the stored sample series representing S_(u)-not signal 510.

[0074] Alternatively, S_(u) signal 502 and S_(u)-not signal 504 are cyclic signals and modem 28 stores a single cycle of each of the signals, with the phase shift and without the phase shift. During transmission of S_(u) signal 502, modem 28 repeatedly transmits the stored cycle of the S_(u) signal which does not have a phase shift. Thereafter, modem 28 transmits S_(u)-not signal 504, by repeatedly transmitting the stored cycle of the S_(u)-not signal without the phase shift. After transmitting 24 samples of S_(u)-not signal 504, modem 28 optionally transmits an interface symbol formed of fractions of both of signals 504 and 506, and then repeatedly transmits the stored cycle of the S_(u) signal, with the phase shift. After receiving J_(p) signal 508, client modem 28 transmits the stored cycle of the S_(u)-not signal with the phase shift.

[0075] Further alternatively or additionally, modem 28 manages a table which includes all the possible samples of the training signals, e.g., S_(u) and/or S_(u)-not, with a phase shift and without a phase shift. The S_(u) signal and the S_(u)-not signal, for example, each has six samples in each cycle, such that the table optionally includes 12 samples for each of the signals, six without the phase shift and six with the phase shift. Optionally, samples at different positions in the cycle which have the same value appear in the table only once. Alternatively or additionally, as the S_(u)-not signal is a mirror image of the S_(u) signal, a single set of values is used for both signals. In generating the training signal, the modem keeps track of the current position in the cycle, whether the S_(u) or S_(u)-not signal is transmitted and whether the phase shift is required. Each time a sample is to be generated, the table is optionally accessed based on the position in the cycle, which signal is transmitted and whether the phase shift is required.

[0076] Referring back to FIG. 5, after the modem training is completed, a modem connection (80) is established and data is transmitted (82) on the connection.

[0077] In some embodiments of the invention, during the data transmission, the entire transmission and reception paths defined for analog modems are used by client modem 28, in addition to an A/D converter which converts the analog generated signals into digital samples, and a D/A converter for the opposite direction. Optionally, one or more of the units of client modem 28 which perform tasks not required for digital connections have a reduced complexity, e.g., use a shorter filter length than generally used.

[0078] Alternatively, for one or more protocols in at least one direction, substantially identical procedures from other protocols, which do not differentiate between analog and digital connections (e.g., V.34), are used. For example, the transmission path of the upstream of the V.90 protocol may be implemented as a V.34 transmission path, such as described in the above mentioned PCT application PCT/IL00/00198.

[0079] Further alternatively, as is now described with reference to FIGS. 8 and 9, client modem 28 includes, for one or more protocols, fewer units than required for an analog connected modem.

[0080] Referring back to FIG. 3A, it is noted that receiving path 40 of an analog modem known in the art may be viewed as formed of two major parts. A first part, an analog to digital conditioning unit, comprises units 44, 46, 48, 50, 52 and 54, which are used to overcome analog signal effects. The analog to digital conditioning unit brings the analog signals received on line 42 substantially back to the state at which they were before the conversion performed by line card 16 (FIG. 2). A second part, a digital interpretation unit, comprises units 56, 58 and 60. The digital interpretation unit translates the digital signals provided by the analog to digital conditioning unit into a form tangible by computer 10.

[0081]FIG. 8 is a schematic block diagram of a receiving path 90 of modem 28, in accordance with an embodiment of the present invention. Optionally, modem 28 receives digital signals on an input line 88 and passes the digital signals directly to a digital interpretation unit 86. Optionally, the digital signals received on input line 88 are passed directly to symbol decision module 56 of digital interpretation unit 86. In some embodiments of the invention, symbol decision module 56 performs detection of attenuation pad impairments and robbed bit signaling (RBS) in addition to determining the samples which represent the symbols. As the downstream digital signals from ISP modem 24 were not converted by a line card 16 (FIG. 2) to analog signals, modem 28 optionally does not perform the tasks of the analog to digital conditioning unit of path 40, i.e., modem 28 does not perform echo cancellation (44, FIG. 3A), and does not pass the signals through channel filter 46, timing recovery module 48, rate converter 50, AGC unit 52 and equalizer 54 (FIG. 3A). Thus, modem 28 is simpler and has a lower CPU and power consumption than modems which would implement all the analog reception tasks. Reducing the CPU consumption of a single modem, allows increasing the number of modems implemented by a single processor of a modem pool.

[0082]FIG. 9 is a schematic block diagram of a transmission path 200 of client modem 28, for transmission of data in accordance with the V.92 protocol, in accordance with an embodiment of the present invention. It is noted that the principles described with reference to transmission path 200 are not limited to the V.92 protocol and may be used for other protocols which differentiate between analog and digital modems. Transmission path 200 optionally includes modulus encoder 63, convolution encoder 66 and inverse mapper 65, as described above with reference to transmission path 61 (FIG. 3B). Transmission path 200, however, does not include a prefilter 67 (FIG. 3B) as such a filter is required only for the analog connection of the modem. In addition, as is known in the art, precoder 64 (FIG. 3B) of transmission path 61 can be viewed as formed of a constellation point selector and a precoder filter. In some embodiments of the invention, as shown in FIG. 9, transmission path 200 includes only a constellation point selector 202 and does not include a precoder filter, which is required only for an analog modem.

[0083] Alternatively, to not implementing the filters, the filters are implemented using degraded values which require less processing resources and perform only partial filtering or no filtering at all.

[0084] Optionally, instead of the filters not used (or degraded), transmission path 200 includes a converter 210, which converts 16 bit linear level samples, as provided by constellation point selector 202, into PCM samples. Optionally, the conversion is performed by finding, for each 16 bit linear level sample the closest level in the 256 scale.

[0085] It is noted that the units of modem 28 shown in FIGS. 8 and 9 may be implemented in hardware, software on one or more processors and/or in any combination thereof. The illustration of the units of modem 28 as different blocks is for clarity of the explanation only.

[0086] It will be appreciated that the above described methods may be varied in many ways, including, changing the order of steps, and/or performing a plurality of steps concurrently. For example, the determination of whether the ability message transmitted by the client modem should indicate analog or digital support may be performed at any time until the actual transmission of the message, including before or after the establishment of the physical connection with the remote modem. It should also be appreciated that the above described description of methods and apparatus are to be interpreted as including apparatus for carrying out the methods, and methods of using the apparatus. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art. Furthermore, the terms “comprise,” “include,” “have” and their conjugates, shall mean, when used in the claims, “including but not necessarily limited to.”

[0087] It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims. 

1. A method of transmitting data, comprising: establishing a modem connection between two digitally connected modems; and transmitting data on the modem connection in accordance with a transmission scheme defined only for transmission from analog to digital modems.
 2. A method according to claim 1, wherein transmitting the data comprises transmitting in accordance with the upstream transmission scheme of the V.92 protocol.
 3. A method according to claim 1 or claim 2, wherein the transmission scheme defined only for transmission from analog to digital modems is defined for use concurrently on a same line with an opposite direction transmission scheme which has a higher maximal transmission rate.
 4. A digitally connected modem, comprising: a digital line interface adapted to transmit symbols at a predetermined rate defining respective symbol periods; and a signal generation unit adapted to generate, during a training phase of a modem connection, symbols of a pair of instances of a training signal, shifted relative to each other by a phase shift of a fractional symbol period, for transmission by the line interface.
 5. A modem according to claim 4, wherein the training signal comprises a signal defined by the V.92 protocol.
 6. A modem according to claim 4 or claim 5, wherein the phase shift comprises a shift of half the symbol period.
 7. A modem according to any of claims 4-6, wherein the signal generation unit comprises an interpolator adapted to convert a generated training signal into a series of samples with a sampling rate higher than that of the signals transmitted by the digital line interface and a decimator adapted to down-sample the training signal to the sampling rate of the signals transmitted by the digital line interface, with a phase shift, when a phase shift is required.
 8. A modem according to any of claims 4-7, wherein the signal generation unit comprises a memory which stores segments of the training signal with and without the phase shift and the generation of the signals is performed with reference to the segments stored in the memory.
 9. A modem according to any of claims 4-7, wherein the signal generation unit comprises a memory which stores a sample series representing both the instances of the training signal together with at least one signal transmitted between the instances.
 10. A modem according to any of claims 4-7, wherein the signal generation unit comprises a memory which stores a table which includes all the sample values of the training signal with different phase shift values.
 11. A modem according to any of claims 4-10, wherein the different instances of the training signal have different lengths.
 12. A modem according to any of claims 4-11, wherein the different instances of the training signal are shifted relative to each other by a shift period including a non-integer number of symbol periods, the phase shift being equal to a fractional portion of the shift period.
 13. A method of transmitting modem training signals, comprising: transmitting, from a first modem to a second modem, a message including a value of at least one parameter of a training signal expected to be received by the first modem; and transmitting, by the second modem, the training signal expected to be received by the first modem, with a predetermined value of the at least one parameter, which is independent of the value of the at least one parameter in the message.
 14. A method according to claim 13, wherein transmitting the message comprises transmitting a J_(p) message of the V.92 protocol.
 15. A method according to claim 13 or claim 14, wherein the at least one parameter is related to a length of the training signal.
 16. A method according to any of claims 13-15, wherein the second modem comprises a digital modem.
 17. A method of establishing a modem connection, comprising: establishing a physical connection between a pair of digital modems; selecting a protocol which requires that at least one of the modems of the connection is a digital modem, to govern the transmission on the physical connection; and transmitting, on the established connection, at least one signal defined for evaluating analog effects of the physical connection.
 18. A method according to claim 17, wherein selecting a protocol comprises selecting a protocol which uses different provisions for upstream and downstream transmission.
 19. A method according to claim 17 or claim 18, wherein transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for evaluating a phase shift caused by the physical connection.
 20. A method according to any of claims 17-19, wherein transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for determining a required echo cancellation.
 21. A method according to any of claims 17-20, wherein transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for channel equalizer training.
 22. A modem, comprising: a line interface adapted to receive signals transmitted in a downstream of a modem connection in accordance with a modem protocol which has different provisions for the upstream and the downstream; and a digital interpretation unit adapted to receive samples from the line interface, after passing through fewer than all of an echo canceller, an automatic gain controller, a rate converter, a time recovery unit, an equalization unit and a channel filter.
 23. A modem according to claim 22, wherein the line interface is adapted to receive signals transmitted in a downstream of a modem connection in accordance with the ITU V.90 or V.92 protocol.
 24. A modem according to claim 22 or claim 23, wherein the digital interpretation unit is connected directly to the line interface.
 25. A modem according to any of claims 22-24, wherein the digital interpretation unit comprises a symbol decision module adapted to receive samples directly from the line interface.
 26. A modem according to any of claims 22-25, wherein the line interface is adapted to receive symbols at a rate of 8000 symbols per second.
 27. A method according to any of claims 22-26, wherein the line interface is connected to a digital trunk line.
 28. A method according to any of claims 22-27, wherein the digital interpretation unit is adapted to receive samples from the line interface, without the samples passing through any unit for overcoming analog signal effects.
 29. A modem, comprising: a line interface adapted to transmit signals in accordance with an upstream of a modem protocol which has different provisions for the upstream and the downstream, on a communication line; and a digital transmission unit adapted to transmit samples through the line interface, without passing the samples through both a precoder filter and a prefilter.
 30. A modem according to claim 29, wherein the digital transmission unit is adapted to transmit samples without passing through a precoder filter or a prefilter.
 31. A modem according to claim 29 or claim 30, wherein the line interface is adapted to transmit signals in accordance with the upstream of the V.92 modem protocol.
 32. A modem according to any of claims 29-31, wherein the digital transmission unit comprises a constellation point selector which is connected to the line interface without intervening filters.
 33. A modem according to any of claims 29-32, wherein the digital transmission unit performs calculations using 16 bit linear level samples and the modem comprises a converter, which converts 16 bit linear level samples into PCM samples.
 34. A modem according to any of claims 29-33, wherein the line interface is adapted to transmit signals onto a digital trunk.
 35. A method of forming a modem connection, comprising: establishing a physical connection between a first and a second modem; and transmitting a message including data on the capabilities of the first modem, from the first modem to the second modem, the message purposely identifying the first modem as being of a different type, having a different capability or being connected differently than the first modem is actually connected.
 36. A method according to claim 35, wherein the first modem is digitally connected and the message identifies the first modem as being analog connected.
 37. A method according to claim 36, wherein the message identifies the first modem as supporting an all digital protocol.
 38. A method according to claim 36, wherein the message does not identify the first modem as supporting an all digital protocol.
 39. A method according to any of claims 35-38, wherein the second modem is digitally connected.
 40. A method according to any of claims 35-39, wherein the message identifies the first modem as supporting at least one protocol which differentiates between analog connected and digital connected modems.
 41. A method according to claim 40, wherein the message identifies the first modem as supporting at least one of the V.90 and V.92 protocols.
 42. A method according to any of claims 35-41, comprising receiving a user instruction on whether to identify as an analog or digital connected modem and wherein the message identifies the connection of the modem responsive to the user indication.
 43. A method according to any of claims 35-42, comprising determining whether to identify as analog or digital connected responsive to a telephone number of the second modem and wherein the message identifies the connection of the modem responsive to the determination.
 44. A method according to any of claims 3543, wherein the message is transmitted during a protocol negotiation procedure.
 45. A method according to claim 44, wherein the first modem does not perform, during the protocol negotiation procedure, at least one test required to determine analog connection parameters.
 46. A method according to claim 44, wherein the first modem disregards results of at least one test required to determine analog connection parameters. 